Problem: manually entering multiple discount codes at checkout doesn't work
A discount code entered by the customer at checkout will override the one provided by Discount Ninja. This is a limitation of Shopify's checkout system.
Solution: enter extra discount codes on the cart page
Discount Ninja allows you to automatically apply multiple discount codes. The app handles the "stacking" of the discount codes as documented here.
However, you may want allow your customers to add manual discount codes. Discount Ninja provides that capability via a custom "Enter discount code" box on the cart page.
Note: this feature requires the Silver plan or up.
The code below adds the following to your cart page:
Add the following code block in the cart template of your theme to add this box:
<input type="text" class="limoniapps-discountninja-cart-discountcode-input" value="" placeholder="Enter discount code">
<input type="button" class="limoniapps-discountninja-cart-discountcode-apply" value="Apply">
<div style="display:none" class="limoniapps-discountninja-cart-discountcode-searching">Searching...</div>
<div style="display:none" class="limoniapps-discountninja-cart-discountcode-applied">Discount code found, it will be applied at checkout.</div>
<div style="display:none" class="limoniapps-discountninja-cart-discountcode-notfound">Discount code not found.</div>